Radar image study of simulated breaking waves

Radar images of electromagnetic scattering from one-dimensional simulated ocean breaking waves are described. Backscatter results from 10-14 GHz at 60/spl deg/ to 80/spl deg/ incident angles are considered for surfaces that satisfy an impedance boundary condition. The generalized forward-backward method with spectral accelerations was used as an exact numerical solution to obtain backscatter returns from several surface profiles, and radar images are formed through back-projection tomography. Detailed investigations of the images are provided to clarify major and secondary scattering events, as well as the polarization dependence, and a ray-tracing analysis is performed to interpret multipath scattering mechanisms. By adding surface roughness outside the breaking region, small-scale roughness scattering effects are also investigated.     

A numerical study of low-grazing-angle backscatter from ocean-likeimpedance surfaces with the canonical grid method

A numerical study of 14-GHz low-grazing-angle (LGA) backscattering from ocean-like surfaces described by a Pierson-Moskowitz spectrum is presented. Surfaces rough in one dimension are investigated with Monte Carlo simulations performed efficiently through use of the canonical grid expansion in an iterative method of moments. Backscattering cross sections are illustrated at angles from 81° to 89° from normal incidence under the impedance boundary condition (IBC) approximation with the efficiency of the numerical model enabling sufficiently large profiles (8192 λ) to be considered so that angular resolution problems can be avoided. Variations with surface spectrum low-frequency cutoff (ranging over spatial lengths from 175.5 m to 4.29 cm) at 3 m/s wind speed are investigated and initial assessments of the small perturbation method (SPM), composite surface theory, operator expansion method (OEM), small slope approximation (SSA), and curvature corrected SPM predictions are performed. Numerical results show an increase in horizontal (HH) backscatter returns as surface low-frequency content is increased while vertical (VV) returns remain relatively constant, as expected, but none of the approximate models considered are found to produce accurate predictions for the entire range of grazing angles. For the cases considered, HH scattering is always observed to be below VV, further demonstrating the importance of improved hydrodynamical models if “super-event” phenomena are to be modeled     

Statistical characteristics of simulated radar imagery from baresoil surfaces: effects of surface roughness and soil moisturevariability

The potential of high-resolution radar imagery to estimate various hydrological parameters, such as soil moisture, has long been recognized. Image simulation is one approach to study the interrelationships between the radar response and the underlying ground parameters. In order to perform realistic simulations, the authors incorporated the effects of naturally occurring spatial variability and spatial correlations of those ground parameters that affect the radar response, primarily surface roughness and soil moisture. Surface roughness and soil moisture images were generated for a hypothetical 100×100 m bare soil surface area at 1 m resolution using valid probability distributions and correlation lengths. These values were then used to obtain copolarized radar scattering coefficients at 2 GHz (L band) and 10 GHz (X band) frequencies using appropriate backscatter models, which were then converted to a digital number within 0-255 gray scale in order to generate radar images. The effect of surface roughness variability causes variability in the radar image, which is more apparent under smooth soil conditions. On the other hand, the inherent spatial pattern in soil moisture tends to cause similar patterns in the radar image under rougher soil conditions. The maximum difference between contrast-enhanced mean values of the radar image digital number due to moisture variations occurs at surface roughness values in the 1.5-2.0 cm range     

A radar cross-section model for power lines at millimeter-wave frequencies

The knowledge of radar backscatter characteristics of high-voltage power lines is of great importance in the development of a millimeter-wave wire detection system. In this paper, a very high-frequency technique based on an iterative physical optics approach is developed for predicting polarimetric radar backscattering behavior of power lines of arbitrary strand arrangement. In the proposed scattering model the induced surface current is obtained using the tangent plane approximation in an iterative manner where the first-order current, obtained from the incident wave, is used as the source for the second-order current and so on. The approximation is valid for frequencies where the cable strand diameter is on the order of or larger than the wavelength. It is shown that the copolarized backscatter is dominated by the contribution from the first-order PO currents, whereas the cross-polarized backscatter is generated by the second- and higher order PO currents. Using this model, the effects of radar antenna footprint, surface irregularities, and cable sag (when suspended between towers) on radar backscatter are studied. To verify the validity of the proposed model, theoretical results are compared at 94 GHz with experimental results and are found to be in good agreement.     

Intercomparison of microwave radiative transfer models forprecipitating clouds

An intercomparison of microwave multiple scattering radiative transfer codes used in generating databases for satellite rainfall retrieval algorithms has been carried out to ensure that differences obtained from retrieval techniques do not originate from the underlying radiative transfer code employed for the forward modeling. A set of profiles containing liquid water and ice contents of cloud and rain water as well as snow, graupel and pristine ice were distributed to the participants together with a black box routine providing Mie single scattering, atmospheric background absorption and surface emissivity. Simulations were to be carried out for nadir and off-nadir (53.1°) observation angles at frequencies between 10 and 85 GHz. Among the radiative transfer models were two-stream, multiple stream and Monte Carlo models. The results showed that there were two major sources of differences between the codes. 1) If surface reflection/emission was considered isotropic, simulated brightness temperatures were significantly higher than for specular reflection and this effect was most pronounced at nadir observation and over ocean-type surfaces. 2) Flux-type models including delta-scaling could partially compensate for the errors introduced by the two-stream approximation. Largest discrepancies occurred at high frequencies where atmospheric scattering is most pronounced and at nadir observation. If the same surface boundary conditions, the same multiple-stream resolution and the same scaling procedures are used, the models were very close to each other with discrepancies below 1 K     

Lessons learned from full-spectral modeling of signatures of an estuary front

Previously, we have modeled radar signatures, involving large variations (/spl sim/10-15 dB) in radar cross section (RCS), that have been observed at strongly convergent ocean fronts and at an estuarine front. In each of these cases, we obtained quantitative agreement with measurement but only by including wave-breaking (WB) effects in an approximate manner. However, in each case, we used the composite scattering (CS) model at a frequency (9.4 GHz) where this model may be deficient. For this reason, questions remain concerning the importance of WB effects in these simulations. In the present study, we monitor the sensitivity of the simulations with respect to this CS approximation by comparing the results of this model with those from an alternative theory, based on simulations of RCS, derived from a common wave spectrum. The spectrum is calculated using a full-spectral treatment of wave-current interaction. The resulting simulations are used to model the radar signature of the buoyant plume associated with the efflux of fresh water from the Chesapeake Bay that was observed during the Chesapeake Outflow Plume Experiment 2 (COPE-II). In both cases, it is possible to simulate this signature, in quantitative agreement with experiment, but only by including WB effects. We find that the CS model predictions for the behavior of the signature do not agree with the comparable predictions from the remaining model. Additional simulations indicate the difference between the two models, which is rather large because of the large (60/spl deg/) angle of incidence, occurs because the CS model includes higher order terms that are not included in the Kirchoff Approximation.     

Two-dimensional and full polarimetric imaging by a syntheticaperture FM-CW radar

Applies the principle of radar polarimetry to a synthetic aperture frequency-modulated continuous wave (FM-CW) radar and presents results based on two-dimensional (2D) full polarimetric imaging. It is shown that the polarimetric target reflection coefficients obtained by the synthetic aperture FM-CW radar are elements of a Sinclair scattering matrix, although the coefficients are derived from a wide band signal. Using the scattering matrix optimization procedure, a 2D polarimetric imaging experiment (including Co-Pol maximum, minimum, span, and phase imaging) of an orthogonally placed linear target set was successfully carried out in the laboratory. This result demonstrates the validity of N-band (8.2-9.2 GHz) FM-CW radar polarimetry, and it presents a demonstration of a full polarimetric 2D FM-CW imaging radar system     

A numerical model for electromagnetic scattering from sea ice

A numerical model for scattering from sea ice based on the finite difference time domain (FDTD) technique is presented. The sea ice medium is modeled as consisting of randomly located spherical brine scatterers with a specified fractional volume, and the medium is modeled both with and without a randomly rough boundary to study the relative effects of volume and surface scattering. A Monte Carlo simulation is used to obtain numerical results for incoherent υυ backscattered normalized radar cross sections (RCSs) in the frequency range from 3 to 9 GHz and for incidence angles from 10° to 50° from normal incidence. The computational intensity of the study necessitates an effective permittivity approach to modeling brine pocket effects and a nonuniform grid for small scale surface roughness. However, comparisons with analytical models show that these approximations should introduce errors no larger than approximately 3 dB. Incoherent υυ cross sections backscattered from sea ice models with a smooth surface show only a small dependence on incidence angle, while results for sea ice models with slightly rough surfaces are found to be dominated by surface scattering at incidence angles less than 30° and by scattering from brine pockets at angles greater than 30°. As the surface roughness increases, surface scattering tends to dominate at all incidence angles. Initial comparisons with measurements taken with artificially grown sea ice are made, and even the simplified sea ice model used in the FDTD simulation is found to provide reasonable agreement with measured data trends. The numerical model developed ran be useful in interpreting measurements when parameters such as surface roughness and scatterer distributions lie outside ranges where analytical models are valid     

Comparison of asymptotic backscattering models (SSA, WCA, and LCA) from one-dimensional Gaussian ocean-like surfaces

In this paper, recent asymptotic backscattering models are compared for one-dimensional multiscale dielectric sea surfaces with Gaussian statistics and by considering the Elfouhaily et al. height spectrum. We focus on the calculations of the normalized radar cross sections (NRCS) obtained from the weighted and local curvature approximations (WCA and LCA), recently published by Elfouhaily, and of the first- and second-order small slope approximation (SSA(1) plus SSA(2) denoted as SSA), developed by Voronovich. Voronovich et al. (2002 Waves Random Media 11 247-269) presented simulations of the SSA by making an assumption over the SSA(2) contribution (the model is referred to as SSAM). The NRCS computation is then similar to SSA(1), where the sea spectrum is substituted by a modified spectrum defined as the product of the sea spectrum by the second-order polarization term. The second-order statistical moment of WCA is calculated rigorously for any two-dimensional height correlation of the surface with Gaussian statistics. The NRCS of the WCA, WCAQ (obtained from a quadratic approximation of WCA), LCA, SSA, and SSAM backscattering models are compared for moderate wind speeds, for microwave frequencies and for backscattering angles ranging from 0 (nadir) to 70/spl deg/.     

Numerical study on the along-track interferometric radar imagingmechanism of oceanic surface currents

The phase information in along-track interferometric synthetic aperture radar (along-track INSAR, ATI) images is a measure of the Doppler shift of the backscattered signal and thus of the line-of-sight velocity of the scatterers. It can be exploited for oceanic surface current measurements from aircraft or spacecraft. However, as already discussed in previous publications, the mean Doppler frequency of the radar backscatter from the ocean is not exclusively determined by the mean surface current, but it includes contributions associated with surface wave motion. The authors present an efficient new model for the simulation of Doppler spectra and ATI signatures. The model is based on Bragg scattering theory in a composite surface model approach. They show that resulting Doppler spectra are consistent with predictions of an established model based on fundamental electrodynamic expressions, while computation times are reduced by more than one order of magnitude. This can be a key advantage with regard to operational applications of ATI. Based on model calculations for two simple current fields and various wind conditions and radar configurations, they study theoretical possibilities and limitations of oceanic current measurements by ATI. They find that best results can be expected from ATI systems operated at high microwave frequencies like 10 GHz (X band), high incidence angles like 60°, low platform altitude/speed ratios, and vertical (VV) polarization. The ATI time lag should be chosen long enough to obtain measurable phase differences, but much shorter than the decorrelation time of the backscattered field     

空间目标的短时三维几何重构方法

通常空间自旋目标的3维(3D)重构都是通过对散射点轨迹进行矩阵分解的方法得到的,散射点轨迹是从雷达序列图提取并关联得到的。由于散射点提取与关联误差的存在,3D重构会出现精度下降,甚至失败的问题。另一方面,转台目标的散射点轨迹符合圆属性,这与几何投影理论认为散射点投影轨迹的椭圆属性相违背。为解决以上问题,该文提出了基于短时的空间目标3D重构算法。首先对提取的散射点轨迹进行2维圆属性拟合,使其轨迹光滑,更接近理论曲线。然后采用多视角的方法估计雷达视角(LOS),通过乘以雷达视角构成的系数,将圆属性轨迹曲线转变成椭圆属性轨迹曲线。通过对散射点椭圆属性轨迹进行矩阵分解的方法获得目标的3D结构。最后通过2个实验验证了该文所提算法的有效性。     

Aircraft measurements of the microwave scattering signature of the ocean

Microwave scattering signatures of the ocean have been measured over a range of surface wind speeds from 3 m/s to 23.6 m/s using the AAFE RADSCAT scatterometer in an aircraft. Normalized scattering coefficients are presented for vertical and horizontal polarizations as a function of incidence angle (nadir to55deg) and radar azimuth angle (0degto360deg) relative to surface wind direction. For a given radar polarization, incidence angle, and azimuth angle relative to the wind direction, these scattering data exhibit a power law dependence on surface wind speed. The relation of the scattering coefficient to azimuth angle obtained during aircraft circles (antenna conical scans) is anisotropic and suggests that microwave scatterometers can be used to infer both wind speed and direction. These results have been used for the design of the Seasat-A Satellite Scatterometer (SASS) to be flown in 1978 on this first NASA oceanographic satellite.     

Determination of volume and surface scattering from saline iceusing ice sheets with precisely controlled roughness parameters

Experiments were performed at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH, to precisely determine the relative contributions of surface and volume scattering from saline ice that has well-known surface roughness characteristics. The ice growth phase of the experiment made use of two 6-ft diameter tanks and a 6-ft diameter mold with known roughness statistical parameters of rms height=0.25 cm and Gaussian correlation (correlation length=2.0 cm). One tank was used for growing a moderately thick saline ice sheet with very smooth surface, and the other was used for growing a thin layer of freshwater ice over the surface mold. The latter resulted in a layer with one statistically known rough boundary and one smooth boundary. Wide-bandwidth, multiple incidence angle backscattering measurements were performed, first on the bare saline ice sheet and then on the same sheet after the thin freshwater ice sheet was placed on top of it. Results indicate that the surface scattering dominates over saline ice volume scattering at all frequencies for low incidence angles for both the very smooth and Gaussian rough surfaces. The significance of volume scattering depends strongly on angle of incidence, frequency, volume scattering albedo, surface roughness, and surface correlation function     

The SIR-B Observations of Microwave Backscatter Dependence on Soil Moisture, Surface Roughness, and Vegetation Covers

An experiment was conducted from an L-band syntheticaperture perture radar aboard space shuttle Challenger in October 1984 to study the microwave backscatter dependence on soil moisture, surface roughness, and vegetation cover. The results based on the anlyses of an image obtained at 21° incidence angle show a positive correlation between scattering coefficient and soil moisture content, with a sensitivity comparable to that derived from the ground radar measurements [1]. The surface roughness strongly affects the microwave backscatter. A factor of 2 change in the standard deviation of surface roughness height gives a corresponding change of about 8 dB in the scattering coefficient. The microwave backscatter also depends on the vegetation types. Under the dry soil conditions, the scattering coefficient is observed to change from about -24 dB for an alfalfa or lettuce field to about -17 dB for a mature corn field. These results suggest that observations with a synthetic-aperture radar system of multiple frequencies ies and polarizations are required to unravel the effects of soil ture,oisre, surface roughness, and vegetation cover.     

Polarimetric scattering from natural surfaces at 225 GHz

Polarimetric radar measurement at 225 GHz which demonstrate that the normalized Mueller matrix may be approximated in terms of a single parameter are presented. The depolarization ratio, defined as the ratio of cross-polarized to co-polarized normalized radar cross section, is shown to accurately predict backscatter from vegetation (trees and grass), as well as scattering from terrain at low incidence angles (snowcover and sand). The depolarization ratio also predicts the standard deviation of the scattered field ellipse parameters     

Effect of multiple scattering on the phase signature of wet subsurface structures: applications to polarimetric L- and C-band SAR

We propose a two-layer integral equation model (IEM) model including multiple-scattering terms to reproduce the phase signature of buried wet structures that we observed on L-band synthetic aperture radar (SAR) images. We have good agreement between the extended (single+multiple scattering) IEM model and previous results obtained using a single-scattering IEM model combined with finite-difference time-domain simulations. We show that the multiple scattering not only significantly influences the copolarized phase difference but can also be related to the soil moisture content. In order to assess the validity of our extended model, we performed radar measurements on a natural outdoor site and showed that they could be fairly well fitted to the extended model. A parametric analysis presents the dependence of the copolarized phase difference on roughness parameters (rms height and correlation length) and radar parameters (frequency and incidence angle). Our study also shows that the phase signature should allow detection of buried wet structures down to a larger depth for C-band (3.8 m) than for L-band (2.6 m). This signature could then be used to map subsurface moisture in arid regions using polarimetric SAR systems.     

Spatial and frequency averaging techniques for a polarimetricscatterometer system

Radar scattering signals from distributed targets exhibit fading due to interference associated with coherent scattering from individual scatterers within the resolution volume. Uncertainty in radar measurements which arises as a result of fading is reduced by averaging independent samples. Independent samples are obtained by collecting the radar returns from nonoverlapping footprints (spatial averaging) and/or nonoverlapping frequencies (frequency agility techniques). An improved formulation of fading characteristics for the spatial averaging and frequency agility technique is derived by taking into account the rough surface scattering process. Kirchhoff's approximation is used to describe rough surface scattering. Expressions for fading decorrelation distance and decorrelation bandwidth are derived. Rough surface scattering measurements are performed between L and S bands. Measured frequency and spatial correlation coefficients show good agreement with theoretical results     

Improved Spatial Mapping of Rainfall Events with Spaceborne SAR Imagery

The Seasat satellite acquired the first spaceborne synthetic-aperture radar (SAR) images of the earth's surface, in 1978, at a frequency of 1.275 GHz (L-band) in a like-polarization mode at incidence angles of 23 ± 30. Although this may not be the optimum system configuration for radar remote sensing of soil moisture, interpretation of two Seasat images of Iowa demonstrates the sensitivity of microwave backscatter to soil moisture content. In both scenes, increased image brightness, which represents more radar backscatter, can be related to previous rainfall activity in the two areas. Comparison of these images with ground-based rainfall observations illustrates the increased spatial coverage of the rainfall event that can be obtained from the satellite SAR data. These data can then be color-enhanced by a digital computer to produce aesthetically pleasing output products for the user community. When the methodology for extracting accurate information about soil moisture status from radar data is developed, it will prove useful in a wide variety of agronomic and hydrological investigations.     

Laboratory measurements of polarization radios of wind wave surfaces

Polarization ratios (sigma_{vv}/sigma_{HH}) of wind-generated rough water surfaces are studied experimentally by means of radar backscatter power measurements. The measurements were made at 9.23 GHz with incidence angles between45degand55degfor wind speeds between 3 m/s to 10 m/s. Scattering surface statistics at all wind speeds were also measured by means of a wave height gauge and a laser slope gauge. The polarization ratio data are presented against parameters pertinent to scattering theory, which are obtained from the measured surface statistics. The results are compared to "composite surface" theory calculations for an assumed wave-height spectral density of the formF(k) propto k^{-m}, wheremis an adjustable parameter.